Known satellite mobile telephone systems include the Inmarsat-M system, the IRIDIUM™ system described in, for example EP-A-0365885, the ICO™ system described in, for example, GB-A-2295296 and the ODYSSEY™ system described in, for example, EP-A-0510789. Whilst each of these systems is cellular in nature, they differ from terrestrial systems in that the cells move relative to the Earth's surface because each cell is defined by a beam from an orbiting satellite.
To extend battery life during the idle mode of a satellite mobile telephone, the mobile telephone is desirably powered down for 95% to 98% of the time. Periodically, the mobile telephone wakes up briefly to determine if broadcast control channels from satellites can be received. If so the mobile telephone checks for an incoming call. In idle mode, the mobile telephone needs to know when to hand over to another beam from the same or a different satellite, i.e. when to start listening to another broadcast control channel.
If a mobile telephone does not know the time of the next handover or the new broadcast control channel, it must frequently search a number of frequencies, for example 8. In particular, if on wake-up the broadcast control channel is found to be weak or absent, the mobile telephone has no way of knowing whether the signal is being blocked or interfered with or whether it is now being serviced by another beam.
It is desirable for a mobile telephone to check only two or three broadcast control channel frequencies (one from a primary satellite and one or two from secondary satellites) instead of all possible frequencies. This minimises “on time” during each wake-up while maintaining the desired low duty cycle. Frequent wake-ups are desirable to minimise the time taken to detect incoming calls and the return of signals after an outage.
It is an aim of the present invention to overcome this problem.